In high speed rotating devices such as gas turbines, the vibration characteristics of rotating members play an important role in determining performance and expected life of the machine. To monitor these vibration characteristics, systems have been developed including a pair of Eddy current sensors mounted near each journal surface. The two Eddy current sensors are radially spaced from each other approximately ninety degrees around the circumference of the journal. As the rotating member rotates within the bearing, signals are produced in response to the changing proximity of the member to the Eddy current sensors.
The signals from the sensors are then studied to determine the vibrational characteristics of the particular bearing of interest. One convenient method of studying the vibrational characteristics is to display the signals from the two Eddy current sensors in a Lissajou plot. Using these plots, the designers or test personnel can easily determine the journal displacement amplitude or orbit magnitude of the member rotating within the bearing and the journal kinematic equilibrium position with respect to the geometric center of the bearing. Hence the Lissajou plots represent the actual motion of a journal within a bearing.
In the case of a rotor in a two bearing system in which signals from each bearing are being monitored simultaneously, the Lissajou plots are displayed to determine the relative phase of one orbit with respect to another which represents the mode shape of the rotational vibration of the system. The term mode shape refers to the deflection shape of the rotor when the rotor goes through the critical speed corresponding to the natural frequency of the system.
In the case of a rigid rotor, for example, the potential mode shapes include a bouncing mode and a conical mode. Orbits of the bouncing mode have their major axis in phase and points on the journals along the same axial reference move in phase. In contrast, orbits of the conical mode, while having their major axis in apparent phase, commit the points on the journals along the same axial reference to a one-hundred eighty degree out of phase motion.
Vibration monitors used in such studies must be tested to ensure proper calibration. Similarly, test personnel must be trained in the use of such vibration monitors and in the vibration characteristics of the machines on which they will be performing tests. To date, adequate devices have not been developed to simulate the journal displacement amplitude, the kinematic equilibrium position of a rotor within a bearing, and the mode shape for a rotor turning in a two bearing system.
The present invention is directed to overcoming one or more of the problems set forth above.